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High power and load? Think ‘synchronous’

09 July 2012

Synchronous motors are increasingly the motor of choice in high power, heavy load applications, writes Marek Lukaszczyk. This is because they are able to correct power factor at the motor power supply, thus reducing electrical power consumption and improving electrical efficiency.

Because of the size and nature of the applications in which synchronous motors are used, the energy savings can be considerable, and often result in a fast payback of the motor’s initial capital cost; with subsequent lifecycle savings helping to reduce the user’s operating costs.

rovide high torque and constant speed under load variation, resulting in low operating and maintenance costs. These benefits are put to use in a wide range of applications, including crushers, mills and conveyor belts in mining and quarry operations; fans pumps and compressors in steel plants; pulp and paper extruders; chippers and de-barkers in wood processing; sewage system pumps; large compressors and fans in chemical and petrochemical applications; mills and crushers in cement works; and water injection pumps on floating oil platforms.

Thanks to their higher efficiency, reduced size and higher output rating capacity, synchronous motors are viewed as a suitable replacement for dc motors in high performance applications. Moreover, in some cases, a lower torque can be specified, bringing a positive reduction in motor starting current, fewer problems with the electric supply network when starting, and an overall reduction in mechanical stresses.

Synchronous motors are more efficient and offer greater speed accuracy compared with induction motors, but their design is more complex. The simpler design of the induction motor makes them less expensive than their synchronous counterparts up to 10kW. However, above this figure, the higher efficiency of synchronous motors will ultimately deliver lower operating costs.

The higher efficiency of synchronous motors is a result of their superior ability at converting electrical energy into mechanical power. Moreover, synchronous motors can be designed for high efficiency operation across a broad speed range, providing significant energy saving across a wide variety of loads. And for applications requiring high torque (crushers, extruders, and the like), the breakdown torque of a synchronous motor can be as much as five times rated torque.

An additional benefit of synchronous motors is their ability to improve stability in variable frequency drive (VFD) applications. Synchronous motors with variable speed are recommended for applications with high torque, low speed and wide speed adjusting range. Depending on load and environment characteristics, motor construction for such applications can be with or without brushes. These motors are suitable for operation at any speed, from zero to maximum, maintaining stability independently of load variation – a characteristic that is of paramount importance for equipment such as laminators and plastic extruders, for example. And brushless machines further reduce running costs as they are less maintenance intensive.

Synchronous motors require a dc power supply to energise the field winding (rotor winding), and this is achieved with a brushless rotating exciter (on brushless versions), or via a static exciter with on motors with brushes (static exciters are frequently used on VFD applications). WEG synchronous motors supplied with static exciters are fitted with slip rings and brushes that allow current powering of the rotor poles through slipping contacts. The dc power supply for the poles is derived from an ac/dc converter and static controller.

WEG’s offering
WEG synchronous motors with brushless excitation system are fitted with a rotating exciter, normally installed on the rear of the motor. Depending on motor operation, the exciter either consists of an exciter with dc power supply on the stator, or one with an ac power supply on the stator.
The company manufactures synchronous motors in sizes up to 20MW, for general industrial applications, and also versions for hazardous (flammable) atmospheres, both on and offshore. Hazardous area motors are supplied with different protection levels, such as Ex-n (non-sparking) and Ex-p (pressurised), meeting national and many international standards.

Marek Lukaszczyk is with WEG UK

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